1. Field of the invention
The present invention relates to a device and method for controlling area gradation for a thermal printer, and more particularly to an area gradation control device and method which enables high speed printing with high quality.
The thermal printer is a device for printing data of one screen using a thermal printing head, and can be a color video printer, a facsimile, etc.
Generally, as shown in FIG. 1, a thermal printer comprises a frame memory 2 for storing video signal data one-frame by one-frame and providing R-(red), G-(green), and B-(blue) color signals under the control of a frame memory controller 1, a color corrector 3 for correcting the R, G, and B color signals provided from the frame memory 2 to Y (yellow), M (magenta), and C (cyan) color signals, a gamma corrector 4 for gamma-correcting the Y, M and C color signals provided from the color corrector 3, a line memory 6 for storing and providing the gamma-corrected video signal data one-line by one-line under the control of a line memory controller 5, and an area gradation controller 7 for area-grading the video signal data provided from the line memory 6 and transmitting the area-graded video signal data to a thermal printing head 8.
The area gradation controller is a circuit for controlling area gradation of video signal data by one line (480 pixels), and transmission of strobe pulses, latch signals, clock pulses, etc., to the thermal printing head. As shown in FIG. 2, a conventional area gradation controller 7 comprises data counter 71 for counting the number of data, control signal generator 72 for providing strobe pulses (STROBE), latch signals (LATCH), and clock pulses (CLOCK) to thermal printing head 8, gradation counter 73 for counting the number of gradations, data latch 74 for dividing input data into odd and even data, and gradation comparator 75 for comparing gradations of odd and even data according to gradation data provided from gradation counter 73 and providing compared data to thermal printing head 8.
With reference to FIGS. 2 and 3, pixel data (480 pixels.times.8 bits) provided from line memory 6 are divided into odd and even bits, which are respectively stored in data latch 74. Then, gradation comparator 75 compares odd data D1 and even data D2 with current gradation data (data supplied from the gradation counter 73). As a result of comparison, if the data value is greater than a gradation value, "high" ("1") level data is transmitted to thermal printing head 8, and if not, "low" ("0") level data is transmitted. The above operation is repeatedly performed for every data of 480 pixels, 256 gradations. When transmission of 480 pixel data is completed, data outputted from gradation comparator 75 is latched in thermal printing head 8 by latch signal LATCH outputted from control signal generator 72 and a strobe pulse STROBE is supplied with a low level input, and accordingly, thermal printing head 8 generates heat. At the same time, the above operation is repeated until the 256th (=2.sup.8) gradation by increasing the gradation by 1, thereby completing data print of one line.
FIG. 4 shows an example of data printed by the above-conventional device, and shows a form of printed one-pixel, in case of having 170 data in one pixel of 256 gradations. A one-pixel is divided into 256 sections and the sections (shaded portion) corresponding to gradations of the respective data (=170) are printed.
However, such a conventional area gradation device has a problem of having difficulty in fidelity-reproduction of original colors and deterioration of picture quality, because only a partial portion corresponding to gradations of data from the front of the pixel, not over the overall one-pixel, is printed. Also, in the area gradation method having 256 gradations, a current gradation is compared by being increased from the 1st to 256th gradation, and the compared data is repeatedly read out 256 times and then is transmitted to the thermal printing head, thereby decreasing the printing speed.
Meanwhile, Japanese Patent Laid-open No. Sho 62-84671 discloses a color printer which can provide a stable temperature correction of the joint of respective divided blocks when a two-block divided thermal printing head prints. This color printer comprises an image memory and a line memory for storing a video signal, a thermal printing head divided into two blocks or more, control means for providing a video signal supplied from the line memory to the respective blocks of the thermal printing head, and processing means for correcting data near the border of the block according to a temperature detected by a temperature sensor positioned near the border of the block of the thermal printing head, thereby printing on-line data with time-sharing two times or more.
However, in transmission of one-line data to the thermal printing head by the control means, data corresponding to the number of gradations is transmitted as in the aforementioned conventional area gradation control device. Therefore, such a conventional color printer cannot solve the problems of poor fidelity-reproduction and deterioration of printing speed.